docs: def _get_SRF_phi(self, imt_per):
          """
          Table 7 and equation 19 of 2013 report. NB change in notation,
          2013 report calls this term 'sigma' but it is referred to here
          as phi.
          """
          if imt_per < 0.6:
              srf = 0.8
          elif 0.6 <= imt_per < 1:
              srf = self._interp_function(0.7, 0.8, 1, 0.6, imt_per)
          elif 1 <= imt_per <= 10:
              srf = self._interp_function(0.6, 0.7, 10, 1, imt_per)
          else:
              srf = 1
  
          return srf